Inkjet printing is usually accomplished by expelling droplets of ink from tiny orifices (nozzles) to land on a recording medium, such as paper. The most common technologies to spray ink from a print head are a thermal process and a mechanical process: in the first one ink is vaporized and thus expelled from the print head, while in the second a piezoelectric transducer is used. This mechanism may be used in a variety of applications, such as printers, plotters, copying machines and fax machines.
The print head is part of an ink cartridge, which physically contains the ink in one or more ink reservoir(s). A representative print head contains a series of nozzles from which the drops of ink are sprayed. A channel is provided to connect the ink reservoir(s) to the nozzles.
Ink cartridges come in various combinations, such as separate black and (multi-)colors cartridges, color and black in a single cartridge or even a cartridge for each ink color. Therefore, a plurality of different fluids may be ejected from the same print head. In such a head, typically each fluid is ejected from a group of closely spaced nozzles and the different groups of nozzles are spaced at a greater distance apart. For each group of nozzles a separated channel is present to connect them to the ink reservoir(s).
Typically, print heads are composite structures, including a semiconductor substrate, a polymeric microhydraulic layer and a metallic or plastic plate in which the nozzles are realized, referred in the following as “nozzle plate”.
The bonding of the nozzle plate to the substrate is made using either an adhesive or by bonding the metallic or plastic plate to a polymeric layer in turn bonded to or deposited on the substrate layer. This polymeric layer serves as a barrier layer to avoid for example leakage of ink from one ink channel/nozzles to the other(s) and to define for each channel some functional fluidic parameters.
The micro-hydraulics layer, including the channel(s) connecting the nozzles to the ink reservoir(s) can be realized on the substrate to form an integral part thereof, whilst the nozzles to eject ink are formed in the metallic or plastic plate adhered to the substrate. Alternatively, the ink channels can be formed in the polymeric layer used to bond the nozzle plate to the substrate, or in the nozzle plate itself, in case the latter is made of polymeric material.
Polymeric nozzle plate integrally formed on the semiconductor substrate can be also realized and, in that case, the print head is referred to as monolithic print head.
In the following, unless otherwise specified, the term “substrate” will be used to designate the assembly of the semiconductor substrate and the micro-hydraulics layer.
When the nozzle plate is made of a metallic or plastic plate adhered to the substrate, the adhesion of the nozzle plate to the substrate is obtained at elevated temperature and under pressure. Generally, the substrate and the nozzle plate have different coefficients of thermal expansion, i.e. the materials in which the print head is formed (including the silicon based substrate, the polymeric layers and the nozzle plate) tend to contract and expand at different rates and of different amounts when they are cooled or heated; this is particularly important in case the nozzle plate is metallic. Thermal stresses are thus generated within the print head when it is cooled to room temperature, after assembly of the layers.
These stresses may warp the print head and cause fractures in the same. In addition, the fact that a plurality of different ink channels may be realized on the substrate weakens the substrate structure thereby increasing the probability of breakage if stresses are present.
Moreover, as the tendency in print heads fabrication is to increase the number of nozzles and channels within the same print head, also print head dimensions increase to accommodate on the same print all these structures, and thus the reduction of the stresses becomes of great importance because stresses also depends on the print head overall geometry.
It is known in the art to form strain relief elements on the print head (i.e. in one of the layers forming the same) in order to reduce these stresses induced in the structure.
In the European patent application No. EP 0925932 in the name of Lexmark International, Inc., a inkjet print head structure is disclosed, comprising a semiconductor substrate, a nozzle plate and a polymeric layer disposed there between. The polymeric layer contains expansion void spaces or valleys sufficient to inhibit stresses in the structure during the process of bonding the nozzle plate to the polymeric layer thereby reducing misalignment and warpage problems associated with conventional print head structures.
U.S. Pat. No. 5,988,786 in the name of Hewlett Packard Company relates to a print cartridge for an inkjet printer and more particularly to an articulate orifice membrane for a print head of a print head inkjet cartridge which improves the trajectory and placement of ink drops by providing reduced deformation of the orifices. In order to reduce the stress, an articulation is introduced into the inner surface of the orifice membrane. This articulation enables stress and strain to be concentrated at points away from the orifices, i.e. at regions bound by the ends of the articulations. In the preferred embodiment, the articulations are realized in form of serrations on the inside of the orifice membrane, such as laser ablated grooves.
In the U.S. Pat. No. 6,527,368 in the name of Hewlett-Packard Company, a fluid ejection device comprises a substrate having a first surface, and a fluid slot in the first surface is shown. The device further comprises a fluid ejector formed over the first surface of the substrate, and a chamber layer formed over the first surface. The chamber layer defines a chamber about the fluid ejector, wherein the fluid flows from the fluid slot towards the chamber to be ejected therefrom.
The U.S. Pat. No. 6,820,963 in the name of Hewlett Packard Development Company, L.P., discloses a fluid ejection head, which includes an orifice layer disposed on top of a substrate layer. The fluid ejection head includes a first group of fluid ejection orifices and a second group of fluid ejection orifices formed in the fluid ejection head, wherein the first group of fluid ejection orifices and the second group of fluid ejection orifices are configured to eject two different fluids, and an elongate channel formed in the fluid ejection head, wherein the channel is positioned between the first group of fluid ejection orifices and the second group of fluid ejection orifices in such a location as to inhibit cross-contamination of fluids ejected from the first group of fluid ejection orifices and second group of fluid ejection orifices.
Applicants have noted that the realization of long continuous channels in the orifice plate excessively weakens the overall structure of the nozzle plate or reduces its size, thereby causing problems during the manipulation of the nozzle plate during the print head assembling operation.
In U.S. Pat. Nos. 5,847,725 and 6360439, and in US patent application No. 2002/0041308 all in the name of Hewlett-Packard Company, a thermal ink jet printer head is disclosed, with an orifice layer for defining numerous of orifice apertures and numerous strain relief elements. Each strain relief element is a closed slit between abutting and separable portions of the plate, such that a stress applied to the plate across the strain relief element will tend to open the slit, or cause the edges to move in a direction perpendicular to the plane of the plate, or otherwise provide a thin cross section that deforms more easily, thereby limiting strain in other portion of the plate.
Applicants have noted that the slits which form the strain relief elements are substantially “one-dimensional”, i.e. they extend substantially along one of the longitudinal axis of the metal orifice layer, whereas in the perpendicular direction (the other axis of the metal orifice plate) their thickness is substantially negligible. The slits thus are designed to deform only along a direction perpendicular to their longitudinal axis. In case of print head in which stresses are present also along the axis of the slits, this stress relief elements configuration may not reduce these stresses appropriately.
In the U.S. Pat. No. 6,799,831 in the name of Canon Kabushiki Kaisha, a liquid discharge recording head comprising a substrate on which an energy generated element for generating liquid discharging energy is provided, and an orifice plate which is laminated with the substrate and in which a discharge port corresponding to the generating energy element is provided, and wherein a liquid droplet is discharged in a direction substantially perpendicular to surfaces of the substrate and the orifice plate, and further wherein a flow path is formed between the substrate and the orifice plate, a groove encircling the flow path is formed in the orifice plate, and edge portions of the orifice plate contacted with the groove are formed as saw-shaped portions having a number of minute indentation.
Among the different embodiments described in this patent, in the seventeenth embodiment a number of through holes which encircle the ink flow path are provided on the orifice plate. The through-holes are cylindrical and are formed using a soluble resin layer: a number of small cylinder are formed and after the coat resin layer as the orifice layer is formed, pouring etching liquid from the discharge ports, the soluble resin is removed.
Applicants have observed that the through-holes in the nozzle plate expose a relatively large portion of the underlying substrate to the contact with the outer environment. This is likely to cause corrosion phenomena, which are likely to damage the substrate itself.
In addition, Applicants have noted that the presence of a large number of circular holes results in a significant portion of the nozzle plate having only very thin integral connection elements (between adjacent holes) to connect the adjacent portion together, this causing an excessive weakening of the nozzle plate. Applicants have further noted that in case of such apertures, a large portion of the plate is removed, weakening the overall structure. Indeed, if these apertures having width and length with cross section of the same magnitude, such in case of cylindrical hole, apertures are formed on the free surface of the nozzle plate having a large area compared to their perimeter. These apertures having such a large area may lead to contamination of the ink by external contaminants.